Preheat oven to 425º. In a large bowl, whisk flour, baking powder, granulated sugar, and salt until combined.

Using the large holes of a box grater, grate cold butter over flour mixture and quickly toss with your hands to incorporate.

Using a wooden spoon, make a well in the center and pour in buttermilk. Stir until just beginning to come together, then turn out onto a work surface. Form dough into a rectangle about 1" thick. Starting with short end, fold dough into thirds, like folding a letter. Using a rolling pin, gently pat back to a 1"-thick rectangle. Repeat folding process once more, working fast so butter doesn't melt.

Pat dough back to a 1"-thick rectangle. Using a sharp knife, cut into 8 pieces. Transfer to a large parchment-lined baking sheet, spacing about ½" apart. Brush tops of biscuits with melted butter.

Bake biscuits until flaky and tops are lightly golden, 20 to 22 minutes.

In a medium skillet over medium heat, cook sausage, breaking up into small pieces with a wooden spoon, until no longer pink, about 8 minutes.

Sprinkle with flour and cook, stirring, until fragrant, about 1 minute. While whisking, slowly pour in milk. Bring mixture to a boil, then reduce heat to medium-low and simmer, stirring often, until thickened, 5 to 7 minutes.

Stir in salt and cayenne, then season with lots of black pepper; season with more salt, if needed.

Break open biscuits. Spoon gravy over.